Miniature lenses or microlenses are increasingly in demand for such purposes as coupling light from a laser to an optical fiber and coupling light from an optical fiber to a photodetector. The U.S. patent of Basavanhally, U.S. Pat. No. 5,135,590, granted Aug. 4, 1992, describes a method for arranging optical fibers in a matrix configuration as is required, for example, for free-space switching. Such uses would normally require a matrix array of microlenses for containing light projected from the optical fiber ends. Thus, considerable effort has been made in developing methods for making arrays of microlenses which can be used as a unit, or from which individual lenses can be selected for use.
One promising method for making microlens arrays comprises the steps of forming an array of photoresist elements on a silica substrate, melting the elements to cause them to have curved or dome-shaped upper surfaces and thereafter solidifying the elements. The photoresist elements and the substrate are then subjected to reactive ion etching, that is, etching by a reactive gas in which the reaction is enhanced by applied radio frequency power. The photoresist elements cause differential etching in the substrate such that, after all of the photoresist has been etched, the dome shapes of the original photoresist elements are replicated in the silica substrate.
We have found that the reactive ion etching (RIE) method of making microlens arrays often results in lenses having undesirable aberrations. Such aberrations can result in significant losses of optical energy and can limit the functions of optical apparatus. Accordingly, there is a continuing need in the industry for dependable methods for making high quality microlens arrays.